1. Field of The Invention
The present invention relates to measuring fiber weight in a continuous sheetmaking machine process, and more particularly, to a sensor for monitoring fiber weight on the wire of a sheetmaking machine.
2. State of The Art
In the manufacture of paper using a continuous sheetmaking machine, a web of paper is formed from an aqueous suspension of fibers (stock). Stock is dispersed from a dispensing unit referred to as a headbox onto a traveling mesh wire or fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dry section where steam heated dryers complete the drying process. The sheetmaking machine is essentially a de-watering, i.e., water removal system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction. Furthermore, in general, the elements of the system including the headbox, the web, and those sections just before the dryer are referred to as the "wet end". The "dry end" generally includes the sections downstream from the dryer. Papermaking elements and machines are well known in the art and are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking machines are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
In the art of making paper, sheet properties (such as sheet strength, thickness, and weight) are continually monitored and the sheetmaking machine controlled and adjusted to assure sheet quality and to minimize the amount of finished product that is rejected. This control is performed by measuring sheet properties at various stages in the manufacturing process which most often include basis weight, moisture content, and caliper (i.e., thickness) of the sheet.
Typically, a scanning sensor is used to perform basis weight measurements of the finished sheet at the dry end of the sheetmaking machine. Scanning sensors are known in the art and are described, for example, in U.S. Pat. Nos. 5,094,535, 4,879,471, 5,315,124, and 5,432,353. The basis weight measurements obtained from the scanner are used to control elements in the sheetmaking machine to adjust basis weight, and hence, paper quality.
There are several problems related to scanning type sensors. In particular, to date scanning sensors have been positioned at or in close proximity to the dry end of the sheetmaking machine. One reason for this is that the scanning measurement equipment cannot tolerate the harsh conditions within the wet end of the sheetmaking machine. Since the scanning sensors are located near the end of the paper making process, process variations occurring at the beginning of the sheetmaking machine in the wet end (i.e., on the wire) are not detected until the final paper is scanned at the dry end. As a result, any paper produced within that time period will potentially have degraded quality. Additionally, since scanning sensors obtain fiber or basis weight measurement readings as the sheet travels along the machine, the actual sensor measurements are taken on a diagonal path. As a result, the measurement readings obtained from a scanning sensor are inter-related machine direction (MD) and cross direction (CD) weight readings. In order to decouple the cross and machine direction weight information, the scanned data is often averaged and processed along with other scanned data readings to obtain an approximation of the MD and CD weight measurements of the paper. This processing also takes time, thereby adding to the overall delay in obtaining feedback data for monitoring process variation and adjusting the sheetmaking machine.
What would be desirable is to obtain fiber weight readings closer to the beginning of the sheetmaking process in a reliable manner.